Disclosed herein is an apparatus and method that determines beam delays in a printing device.
Presently, electrophotographic marking is a method of copying or printing documents in a printing system. Electrophotographic marking exposes a substantially uniformly charged photosensitive surface of a photoreceptor to an optical light image of an original document. The photoreceptor is discharged to create an electrostatic latent image of the original document on the photoreceptor's surface. Toner is then selectively adhered to the latent image. The resulting toner pattern is transferred from the photoreceptor either directly to a marking substrate such as a sheet of paper or indirectly to a marking substrate after an intermediate transfer step. The transferred toner powder image is subsequently fused to the marking substrate using heat and/or pressure to make the image permanent. Finally, the surface of the photoreceptor is cleaned of residual materials and recharged in preparation for the creation of another image.
A raster output scanner is one system commonly used for electrophotographic marking. A raster output scanner includes at least one optical emitter, such as a laser beam source. A raster output scanner also includes a means for modulating the resulting laser beam, which, as in the case of a laser diode, may be the action of turning the source itself on and off, such that the laser beam contains image information. A raster output scanner further includes a rotating polygon mirror having one or more reflective surfaces and other optics, such as pre-polygon optics for collimating the laser beam, post-polygon optics for focusing the laser beam into a well-defined spot on the photoreceptor surface and for compensating for a mechanical error known as polygon wobble, and one or more folding mirrors to reduce the overall physical size of the scanner housing. The laser source, modulator, and pre-polygon optics produce a collimated laser beam which is directed to the reflective polygon facets. As the polygon rotates, the reflected beam passes through the post-polygon optics and is redirected by folding mirrors to produce a focused spot that sweeps along the surface of the charged photoreceptor. Since the photoreceptor moves in a process direction that is substantially perpendicular to the scan line, the spot sweeps the photoreceptor surface in a raster pattern. By suitably modulating the laser beam in accordance with the position of the spot, a desired latent image can be produced on the photoreceptor.
Some raster output scanners employ more than one laser beam. Multiple laser beam systems are advantageous in that higher overall process speeds can result if the individual laser beams expose the raster scan lines in parallel at a given resolution, or higher resolution can be provided if the individual laser beams expose multiple raster scan lines at the same process speed. Typically, raster output scanners that employ multiple optical emitters have a parallel path architecture with closely spaced beams. Closely spaced laser beams are beneficial in that they can be arranged to share common optical components including the same polygon facets, the same post-polygon lens, and the same mirror system. This tends to minimize relative misalignment errors caused by manufacturing differences in the optical components.
A phenomenon known as scan line jitter exists in electrophotographic printing. Scan line jitter refers to the failure of pixels in successive scan lines of the raster to be precisely aligned with each other. For example, jitter is a dysfunction, or mis-position noise, of not placing a pixel in the correct position on the photoreceptor surface to create a straight line. To help reduce scan line jitter it is common to position a photodetector element in the scan line path just ahead of the latent image area in order to establish accurate data clock phasing on successive scans, a technique generally referred to as start-of-scan detection. When a laser beam crosses the photodetector, a fast start-of-scan transition or edge is produced which is used to initialize the pixel clock controlling the phase of the data stream that modulates the laser beam.
One problem with using multiple optical sources occurs because the sources, while closely spaced, still cannot be in the same physical location. Because the sources must be located next to each other, their output video must be delayed on a per diode basis for proper alignment, such as in order to form a vertical line on the photoreceptor. If the output video is not delayed properly, the sources will not produce a proper image on the photoreceptor.
Thus, there is a need for an apparatus and method that determines beam delays in a printing device.